This invention generally relates to offshore oil and gas operations and more particularly to deep-water floating oil production, storage, and offloading operations.
As oil and gas operations have moved further offshore into ever deeper water in search of new fields, fixed production platforms have continued to grow in height and expense. Fixed platforms have battered or sloped legs to provide stability, and the weight of steel required increases geometrically with water depth. Shell""s Bullwinkle Platform in about 1400 feet of water was perhaps the deepest conventional platform ever installed. In deeper water, however, the use of bottom-founded steel or concrete structures for oil well drilling and production operations becomes quite expensive due to the high cost of fabrication and installation of such large structures.
As such, it has become more or less common practice to employ floating production structures in developments of oil and gas fields in deeper water offshore. These facilities may take one of several familiar offshore hull forms, for example semi-submersibles, spar-buoys, tension leg platforms, barges, or ships. All of these hull forms are moored, displacement buoyed vessels, and all respond to forces of wind, waves, and current. Because the wells and moorings are fixed at the sea floor, dealing with the motions of the production facility on the water""s surface is at the heart of deepwater production system design.
Ships are the preferred floating production structure in that they are very efficient in terms of cargo and load carrying capacity. Although they are perhaps the most responsive floating production facility type to environmental forces in terms of motions, they have reasonable motion characteristics when headed into the weather and seas. Conversely, ships exhibit relatively poor motions in quartering or beam seas.
When ships are employed in floating production service where the weather and seas are mild and directional, such as offshore West Africa, they are often simply spread moored with flexible risers suspended over the side of the ship, like Exxon Mobil""s Zafiro Producer. In most other areas of the world, though, the ship must be allowed to rotate or xe2x80x9cweathervanexe2x80x9d in order to keep the vessel headed into the weather or seas to reduce station-keeping loads and motions to acceptable levels. Weathervaning may be passive, or the vessel may be equipped to allow the operator to select a heading for the vessel. The latter feature is important where environmental forces are not co-linear, for example, where the wind forces and sea forces are at an angle to each other.
During weathervaning, the ship or vessel must be free to rotate while the moorings and production risers are fixed; i.e., not free to rotate with the vessel, and yet connected to the process system on the vessel. The ability to maintain this connection and yet weathervane is generally accomplished by means of swivels and one of several types of turntables, turrets, or turret transfer systems. The upper end of the vessel mooring cables and risers are connected to the turret, and the lower ends are connected to equipment at the sea floor, so the horizontal orientation of the turret is relatively fixed. The ship is allowed to rotate or weathervane around the turret.
The turret may be fitted in a vertical well through the ship""s hull or it may be connected to the ship by means of an articulating yoke, usually at the bow. The turret may contain anti-friction or plain bearings, but in either case the bearings are required to translate the mooring forces into the hull. Turret rotation with respect to the ship may be active or passive. Propeller thrusters fitted to the ship that essentially rotate the ship around the turret accomplish active vessel rotation. Passive weathervaning involves placing the turret forward of midship and simply allowing environmental forces to rotate the vessel into a heading that minimizes mooring forces.
The well fluids, controls, and mooring loads must be transferred from the turret to the ship. The fluid transfer is accomplished by means of a multi-path high-pressure swivel or less frequently by means of a xe2x80x9cdrag-chainxe2x80x9d carousel-type hose support structure and flexible hoses. The hoses are reeled off of or onto the carousel structure as the vessel rotates around the turret. The latter limits rotation to about 270 degrees in either direction, and so rotation of the ship around the turret must be of the active type. The turret, turret well, bearings, and fluid transfer system are very costly, in some cases about $70 million. Some turret systems are even more expensive because they are designed to be disconnected from the ship under certain circumstances, for example, when icebergs or cyclonic storms may be a problem.
A floating production mooring system in deep-water usually consists of twelve mooring legs, each consisting of two or more miles of between 4 and 6-inch diameter line of chain and wire combination. As is known in the art, the length of each mooring leg or line may be about 2 xc2xd to about 3 xc2xdtimes the water depth. Installation of these mooring lines therefore requires the operational services of another sea-going vessel, such as a working barge or service vessel, which is used to carry the heavy lines from a port to the offshore well site for anchoring. Because of the size and weight of the mooring lines, multiple trips are needed by the working barge and its crew to secure the floating production ship to the sea bed, often over the course of a month or longer. Consequently, such a system is very costly to purchase and install, perhaps $30 million in 5000 feet of water. In addition, because the motions of the ship are translated into the riser system, floating production ships generally employ flexible pipe riser systems connected to subsea trees. The sophisticated flexible pipe risers used today in deep water can cost several million dollars per riser, and as much again for installation.
Accordingly, there is a need for a floating production ship that can quickly become operational and which avoids costly turret systems and mooring legs, uses rigid steel drill pipe risers, and is able to weathervane through at least 90 degrees in either direction, thus allowing the vessel to present a fair form to environmental forces from any direction.
The present invention is directed to a floating production vessel operating on the surface of the sea with two or more risers connected between equipment or wells on the sea floor and the vessel or its equipment at the surface. The risers are rigid, jointed pipe with rugged field proven drill-pipe type connections. The arrangement, spacing, and support of the risers at the vessel and on the sea floor is such that the vessel, riser supports, and upper ends of the risers may be rotated from a neutral heading up to about 90 degrees in either direction without the use of a swivel or turret at the surface or subsea, and without damage to the risers or without reducing the spacing between the risers unacceptably.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily used as a basis for modifying or designing other floating production ships for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth herein.